lecture 6 - membranes - north seattle collegefacweb.northseattle.edu/lizthomas/lecture 6.pdf · in...

Lecture 6 - Membranes

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In this lecture…

• The fluid mosaic model of the plasma membrane

– Components of the membrane

• Active vs. passive transport

• Diffusion and osmosis

• Endocytosis and exocytosis

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The plasma membrane • The boundary that separates the living cell from its

surroundings

• The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others

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However…

• The membrane isn’t just a phospholipid bilayer

• It is composed of a huge array of phospholipids, regular lipids, proteins, and other molecules

• This is called the fluid mosaic model

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Fluid Mosaic Model

• The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it

• Phospholipids and some proteins can drift laterally – Very rarely does someone “flip”

• How do proteins stay embedded?

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Figure 7.5

Glyco- protein

Carbohydrate

Glycolipid

Microfilaments of cytoskeleton

EXTRACELLULAR SIDE OF MEMBRANE

CYTOPLASMIC SIDE OF MEMBRANE

Integral protein

Peripheral proteins

Cholesterol

Fibers of extra- cellular matrix (ECM)

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The components of the membrane

• Phospholipids

• Proteins

• Cholesterol

• Cytoskeletal support

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What are membranes made of? Phospholipids!

• Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions

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Movement within the membrane

An enzyme called “flipase” (really) can catalyse the flipping of lipids

Membrane proteins

Mouse cell Human cell

Hybrid cell

Mixed proteins after 1 hour

RESULTS

How was membrane movement demonstrated?

Some different types of phospholipids

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What factors influence fluidity?

• Membranes start solidifying at cool temps

• The temperature at which a membrane solidifies depends on the degree of saturation in the fatty acid tails

– Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

• Membranes must be fluid to work properly; they are usually about as fluid as salad oil

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Cholesterol • Cholesterol is a “fluidity buffer”

– Restrains phospholipid movement at body temps

– Also hinder close packing, so lowers the temp required for membranes to solidify

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Membrane Proteins

• A membrane’s function is defined by the proteins embedded in it

• Two types of membrane proteins: – Integral protein

• Penetrate the hydrophobic interior; can stick out of the surface

• Integral proteins that span the membrane are called transmembrane proteins

– Peripheral protein • Stick to the surface of the membrane

Figure 7.9

N-terminus

helix

C-terminus

EXTRACELLULAR SIDE

CYTOPLASMIC SIDE

Hydrophobic R groups on the helices

of the interior

Hydrophilic R groups on the water-

facing sides

Integral or peripheral?

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This protein has secondary and tertiary structure, but no quaternary structure

Structure of a membrane protein

Six Abilities of Membrane Proteins

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Many membrane proteins are glycoproteins/proteoglycans

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Membrane carbohydrates in cell-cell recognition

• Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma membrane

• Membrane carbohydrates may be covalently

bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins)

• Carbohydrates on the external side of the plasma

membrane vary among species, individuals, and even cell types in an individual

Membrane proteins in cell-cell recognition

• Membrane proteins help HIV invade immune system cells

• HIV’s gp120 protein binds to human’s CD4 glycoprotein + CCR5 coreceptor

CCR5-Δ32 Mutation • The CCR5-Δ32 mutation deletes the CCR5

coreceptor, preventing HIV from infecting cells

• Present in 10% of people from Northern Europe

• Marrow transplantation from an immune donor confers immunity

– Immune system cells come from bone marrow

– Clinical trials underway to treat HIV+ people with their own genetically engineered marrow

• Those without CCR5 are more susceptible to West Nile Virus

What is a receptor? A coreceptor? What are they made of? How are they part of the cell

membrane?

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The ABO blood group

• Your genes determine what kind of carbohydrate you get on the surface of your red blood cells

• Three types of glycoproteins: A, B, O

• Your body recognizes a “not you” group, gets mad, and destroys the cell bearing it

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The internal cytoskeleton supporting the membrane

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Selective permeability

• Nonpolar molecules can dissolve in the lipid bilayer and pass through indiscriminately – Carbon dioxide, oxygen, hydrocarbons

• Hydrophobic membrane interior prevents polar molecules from easily crossing

• Transport proteins help the helpless – Allow hydrophilic substances to pass through

• So…selective permeability depends both on the lipid bilayer and on the transport proteins

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What is the fluid mosaic model? Describe how the following components fit into the fluid

mosaic model:

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• Phospholipid saturation • Cholesterol • Membrane proteins • Glycoproteins and proteoglycans • Cytoskeleton

Properties of the plasma membrane

• Selective permeability

• Passive transport

– Diffusion

– Osmosis

– Facilitated diffusion

• Active transport

– Exocytosis

– Endocytosis

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Passive vs. active transport across the membrane

• In some cases molecules will spontaneously diffuse across the membrane

– Passive transport

• Other molecules require a protein or other force to bring them across the membrane

– Active transport

Diffusion

• Passive transport

• Molecules tend to diffuse out evenly into available space due to random movements

• A substance will diffuse where it is more concentrated to where it is less concentrated – Diffuse down its diffusion gradient

– “The region along which the density of a chemical substance increases or decreases”

– Spontaneous, no energy input required

Figure 7.13 Molecules of dye

Membrane (cross section)

WATER

(a) Diffusion of one solute

(b) Diffusion of two solutes

Net diffusion Net diffusion

Net diffusion Net diffusion

Net diffusion Net diffusion

Equilibrium

Equilibrium

Equilibrium

Diffusion of water - Osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane

• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides

Figure 7.14 Lower concentration of solute (sugar)

Higher concentration of solute

Sugar molecule

H2O

Same concentration of solute

Selectively permeable membrane

Osmosis

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What happens in cells without walls? Hypertonic or hypotonic environments create osmotic problems for organisms

…and cells with walls?

• Cell walls help maintain water balance

• A plant cell in a hypotonic solution swells until it hits the wall; the cell is now turgid (firm)

• If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt

• In a hypertonic environment, plant cells lose water; the membrane pulls away from the wall, a usually lethal effect called plasmolysis

• (Lysed means the plasma membrane has broken)

If you don’t water your plant…(or yourself)

• Water osmoses out of the plant cells and evaporates, turning the entire plant limp

Reverse osmosis: turning seawater drinkable

• Drawing salt out of seawater is called desalination

• Pressure applied to seawater forces it through a semipermeable membrane

• The membrane allows water through, but not solutes

Osmosis Vocabulary

• Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water

• Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane

• Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water

• Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water

Hypotonic solution

Isotonic solution

Hypertonic solution

(a) Animal cell

(b) Plant cell

H2O H2O H2O H2O

H2O H2O H2O H2O Cell wall

Lysed Normal Shriveled

Turgid (normal) Flaccid Plasmolyzed

The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

Facilitated Diffusion • Transport proteins speed the passive movement of

molecules across the plasma membrane

• Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane.

• Channel proteins include

– Aquaporins, for facilitated diffusion of water • 3 billion water molecules/second

– Ion channels that open or close in response to a stimulus (AKA ion-gated channels)

• Carrier proteins undergo a conformational change that brings the molecule from one side to the other

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Active transport

• Uses energy to move molecules against their gradients

• All carrier proteins

• Includes ion pumps and endocytosis/exocytosis

The electrochemical gradient and ion pumps

• All cells have voltages across their membranes – Voltage is electrical potential energy – a separation of

two opposite charges

– Called a membrane potential

– Inside of the cell is -50 to -200millivolts because of a higher proportion of anions:cations

– “The cytoplasmic side is negative relative to the outside because of unequal distribution of cations and anions”

• The membrane potential favors cations passively diffusing across – Anions must use active transport

• So two forces actually drive the diffusion of ions across a membrane: the concentration gradient and electrical force – Does not apply to solutes like glucose, only charged

ions

• Combination of the two forces is called the electrochemical gradient

• Cells create an electrochemical gradient by pumping ions inside or outside the cell

• The electrochemical gradient can be taken advantage of to perform chemical reactions – Convert electrical potential energy to chemical

potential energy

Channel protein

Cotransport: changing between different types of potential energy

• A substance that has been pumped across the membrane can do work as it moves back across by diffusion – Water pumped uphill that spins a turbine as it

flows back down

• A cotransport protein can couple the “downhill” passive diffusion to a second “uphill” active transport of a different substance

Final result: expending ATP to bring in sucrose What a cotransporter actually looks like (the Na-K-Cl transporter)

Cotransporters in diarrhea • Bodily fluid high in salt is pumped into the

intestine to help digestion

• Normally sodium in waste is reabsorbed in the colon

• With diarrhea, waste is expelled so rapidly sodium cannot be reabsorbed fast enough and levels fall to dangerous levels

• A salt-glucose solution is administered, which is taken up by sodium-glucose cotransporters

• Mortality rate of cholera (causes massive diarrhea) dropped from 70% to 30% after we figured out to administer this hypertonic solution

Type of movement across the membrane

‘Helping’ agent Requires energy?

Diffusion Diffusion gradient No

Facilitated Diffusion Diffusion gradient + transport proteins

Sometimes, in the form of ATP

Active transport Transport proteins Yes, in the form of ATP

Endocytosis and Exocytosis: other forms of active transport

Endocytosis: Engulfing food/foreign material

Exocytosis: Expelling waste products/unneeded material

Ex: nerve cell releasing neurotransmitters

Exocytosis in nerve cells

Endocytosis and Exocytosis

Pinocytosis: “Cellular drinking” A type of endocytosis

Phagocytosis: “Cellular eating” Another type of endocytosis

Pinocytosis in muscle cells

Phagocytosis in white blood cells

Phagocytosis in amoeba

Receptor-mediated endocytosis: Targeted swallowing of material

Receptors on the cell surface membrane only bind to certain things, triggering endocytosis of that specific thing

A summary of the various -cytoses

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Type of Cell Action Description of action

Exocytosis

Endocytosis Pinocytosis

Endocytosis Phagocytosis*

Endocytosis Receptor-mediated endocytosis

Receptor-mediated endocytosis and HIV

• Viruses infect our cells by attaching to cell-surface receptors meant for receptor-mediated endocytosis

– They ‘fool’ the receptor by mimicking the natural protein that would usually bind

– HIV’s gp120 protein binds to a white blood cell’s CD4 and CCR5 protein

• Triggers endocytosis of the HIV virus

Gp120 is a glycoprotein

Structure of gp120

(CCR5)

protein blocking the binding

of gp120 to CCR5,

preventing HIV entry

Vocabulary

• Fluid mosaic model • Selective permeability • Glycolipid, glycoprotein, proteoglycan • Transmembrane proteins • Cholesterol • Active transport, passive transport, facilitated diffusion • Diffusion gradient • Osmosis • Ion channels • Cotransport • Endocytosis, exocytosis

– Phagocytosis – Receptor-mediated endocytosis